Catalyst for production of hydrocarbons

ABSTRACT

A catalyst for converting synthesis gas composed of hydrogen and carbon monoxide to hydrocarbons. The catalyst includes cobalt in catalytically active amounts up to about 60 wt % of the catalyst, rhenium in amounts of about 0.5 to 50 wt % of the cobalt content of the catalyst and an alkali in amounts ranging from about 0.5 to 5 atom percent of the cobalt content of the catalyst, supported on alumina. A metal oxide promoter may be added.

BACKGROUND OF THE INVENTION

This application is a continuation-in-part of Ser. No. 113,095, filedOct. 23, 1987, now U.S. Pat. No. 4,801,573.

FIELD OF THE INVENTION

The present invention relates to catalysts and more particularly to acatalyst for converting synthesis gas to hydrocarbons.

DESCRIPTION OF THE PRIOR ART

The reaction to convert carbon monoxide and hydrogen mixtures (definedherein as synthesis gas or syngas) to higher hydrocarbons over metalliccatalysts has been known since the turn of the century. This reaction iscommonly referred to as the Fischer-Tropsch or F-T synthesis. The F-Tsynthesis was exploited commercially during World War II in Germany. By1944 a total of nine F-T plants were operating in Germany, primarilyusing a catalyst composed of cobalt, magnesium oxide, thorium oxide andkieselguhr, in the relative proportions of 100:5:8:200. Later, most ofthe thoria was replaced by magnesia, primarily for economic reasons.Currently, commercial Fischer-Tropsch plants are operating in SouthAfrica. These plants use a precipitated iron-based catalyst whichcontains various promoters to improve the stability and productdistribution.

The common F-T catalysts are nickel, cobalt and iron. Nickel wasprobably the first substance to be recognized as capable of catalyzingthe reaction of syngas to hydrocarbons, producing mainly methane (see,for example, "The Fischer-Tropsch Synthesis" by R. B. Anderson, AcademicPress (1984), p.2). Iron and cobalt are able to produce longer chainlength hydrocarbons and are thus preferred as catalysts for theproduction of liquid hydrocarbons. However, other metals are alsocapable of catalyzing the F-T synthesis. Ruthenium is a very activecatalyst for the formation of hydrocarbons from syngas. Its activity atlow temperatures is higher than that of iron, cobalt or nickel; and itproduces a high proportion of heavy hydrocarbons. At high pressures, itproduces a high proportion of high molecular weight wax. Osmium has beenfound to be moderately active, while platinum, palladium and iridiumexhibit low activities (see Pichler, "Advances in Catalysis", vol. IV,Academic Press, N.Y., 1952). Other metals which are active, such asrhodium, yield high percentages of oxygenated materials (Ichikawa,Chemtech, 6, 74 (1982)). Other metals that have been investigatedinclude rhenium, molybdenum and chromium, but these exhibit very lowactivities with most of the product being methane.

Various combinations of metals can also be used for synthesis. Dopingcobalt catalysts with nickel causes an increase in methane productionduring F-T synthesis (see "Catalysis", vol. IV, Reinhold Publishing Co.,(1956), p. 29). In U.S. Pat. No. 4,088,671 to T. P. Kobylinski, entitled"Conversion of Synthesis Gas Using a Cobalt-Ruthenium Catalyst", theaddition of small amounts of ruthenium to cobalt is shown to result inan active F-T synthesis catalyst with a low selectivity to methane.Thus, these references teach that the combination of two or more metalscan result in an active F-T catalyst. In general, the catalysts of theseteachings have activities and selectivities which are within the rangesof the individual components.

Combinations of metals with certain oxide supports have also beenreported to result in an improved hydrocarbon yield during F-Tsynthesis, probably due to an increase in the surface area of the activemetal. The use of titania to support cobalt or cobalt-thoria is taughtin U.S. Pat. No. 4,595,703, entitled "Hydrocarbons from Synthesis Gas".In this case the support serves to increase the activity of the metal(s)toward hydrocarbon formation. In fact, titania belongs to a class ofmetal oxides known to exhibit strong metal-support interactions and, assuch, has been reported to give improved F-T activity for a number ofmetals (see, for example, S. J. Tauster et al, Science, 211, 1121(1981)). Combinations of titania and two or more metals have also beenshown to yield improved F-T activity. In U.S. Pat. No. 4,568,663,entitled "Cobalt Catalysts. In the Conversion of Methanol toHydrocarbons and for Fischer-Tropsch Synthesis", combinations of cobalt,rhenium and thoria and cobalt and rhenium supported on titania areclaimed useful for the production of hydrocarbons from methanol orsynthesis gas. This patent also indicates that similar improvements inactivity can be obtained when cobalt-rhenium or cobalt-rhenium-thoria iscompounded with other inorganic oxides. However, titania is the onlysupport specifically discussed. The typical improvement in activitygained by promotion of cobalt metal supported on titania with rhenium isless than a factor of 2. We have found that the addition of rhenium tocobalt metal supported on a number of other common supports results insimilar improvements in activity.

The only other examples in the literature of catalysts involvingmixtures of cobalt and rhenium refer to completely different chemicalreactions. For example, in Soviet Union Patent 610558, a catalystcomposed of cobalt and rhenium supported on alumina is taught to resultin improved performance for the steam reforming of hydrocarbons. Steamreforming of hydrocarbons is a process completely different fromhydrocarbon production via F-T synthesis and is believed to proceed by acompletely different mechanism. Although some steam reforming catalystscan convert synthesis gas to hydrocarbons, such catalysts are notselective for the production of high carbonnumber hydrocarbons (C3 andabove) during conversion of synthesis gas. In fact, most commonly usedsteam reforming catalysts contain nickel as their active metal, andnickel produces mostly methane when used for syngas conversion.

SUMMARY OF THE INVENTION

It has been found in accordance with the present invention thatsynthesis gas comprising hydrogen and carbon monoxide can be convertedto liquid hydrocarbons by using a catalyst consisting of cobalt andrhenium supported on alumina and promoted with an alkali. As usedherein, alkali refers to one or more of the elements lithium, sodium,potassium, rubidium, and cesium of group IA of the periodic table. Thecatalyst preferably contains from about 5 to 60% cobalt, has a rheniumcontent between 0.5 and 50% of the amount of cobalt, and an alkalicontent between 0.5 and 5 atom % of the amount of cobalt. The aluminapreferably is gamma alumina.

It has been found that the addition of small amounts of rhenium tocatalysts consisting predominantly of cobalt supported on aluminaunexpectedly results in greatly enhanced activity of this catalyst forhydrocarbon production from syngas. This is surprising in light of thefact that rhenium supported on alumina shows very low activity, withmost of the product being methane. Furthermore, rhenium addition tocobalt supported on supports other than alumina results in catalystswith much lower activity levels. In addition, the more active cobaltplus rhenium catalyst maintains the high selectivity to higherhydrocarbons and the low selectivity to methane found with analuminasupported cobalt catalyst. It has also been found that theaddition of small amounts of an alkali to these catalysts serves toincrease the average carbon number of the products produced during F-Tsynthesis. Both the high activity and the low methane production ofcobalt-rhenium on alumina are unexpected in light of the facts that (1)rhenium shows very low activity for F-T synthesis, (2) the main productsfrom F-T synthesis over a rhenium catalyst are methane and carbondioxide, and (3) the use of alumina as a support for catalystscontaining only cobalt results in no, or at best only a slight, increasein activity compared to the use of cobalt on other supports. Thus, forreasons not fully understood, the combination of cobalt and rhenium plusan alkali supported on alumina results in a catalyst which issignificantly more active than either of the two individual metalssupported on alumina or the combination of the two metals supported onother inorganic supports, such as silica, magnesia, silica-alumina,titania, chromia or zirconia. Furthermore, the product distribution witha high selectivity to C₂ +hydrocarbons and low selectivity to methaneand carbon dioxide would not have been predicted based on the knownproduct distribution from rhenium catalysts. In addition, the additionof an alkali to the catalyst serves to increase the average carbonnumber of the product. This is advantageous in situations where themarket value of the lighter products is lower than that of the heavierproducts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the effect of rhenium content on CO conversionusing catalysts containing 12% cobalt;

FIG. 2 is a graph showing the effect on CO conversion of adding rheniumto catalysts containing various amounts of cobalt on an alumina support;

FIG. 3 is a graph showing the effect of potassium to cobalt ratio on theSchulz-Flory α of the product produced using the catalysts of thisinvention; and

FIG. 4 is graph showing the effect of potassium to cobalt ratio on COconversion when using the catalysts of this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The catalyst of the present invention comprises as the active catalyticingredients cobalt, rhenium and an alkali, that is an element from GroupIA of the Periodic Table, supported on alumina with rhenium and thealkali present in relatively smaller amounts than cobalt. This catalysthas been found to be highly active for the conversion of synthesis gas,a mixture of hydrogen and carbon monoxide, into a mixture ofpredominantly paraffinic hydrocarbons. As indicated above, it has longbeen known that cobalt is an active catalyst for the F-T synthesis. Itis also known that the addition of rhenium to a cobalt catalystsupported on titania gives improved activity, even if rhenium by itselfshows very low activity for F-T synthesis and produces methane as themain product. Surprisingly, we have found that the choice of support forthe cobalt plus rhenium catalyst is very critical, and that the additionof rhenium to an alumina-supported cobalt catalyst gives a much higherimprovement in activity than addition of rhenium to cobalt supported onother inorganic oxides. Also, the alkali increases the average carbonnumber of the product.

The cobalt is added to the alumina support in some amount up to about 60wt % of the catalyst, including cobalt. Preferably, amounts between 5and 45 wt % are used; and more preferably between about 10 to about 40wt %. The content of rhenium is between about 0.5 and 50 wt % of thecobalt content; preferably between 1 and 30 wt %; and more preferablyfrom about 2 to around 20 wt %. The content of alkali is between about0.5 and 5 atom % of the cobalt content.

In addition to cobalt, rhenium and alkali, it may be beneficial toinclude a small amount of an additional metal oxide promoter in anamount between about 0.1 and 5 wt %, and more preferably between about0.2 and 2 wt %, based on the weight of the complete catalyst. Thepromoter is suitably chosen from elements in groups IIIB, IVB and VB ofthe periodic chart, the lanthanides and actinides. The promoter oxidecan be chosen from, for example, Sc₂ O₃, Y₂ O₃, La₂ O₃, Ce₂ O₃, Pr₂ O₃,ZrO₂, Ac₂ O₃, PaO₂, Nd₂ O₃, CeO₂, V₂ O or Nb₂ O₅. The most preferableoxide is La₂ O₃, or a mixture of lanthanides, rich in lanthanum. Oxideslike MnO or MgO can also be included. While not essential, the use ofthese metal oxides is common in the art, since they are believed topromote the production of products with higher boiling points, whilemaintaining or improving catalytic activity. However, the catalyst ishighly active and selective without the addition of one or more of thesemetal oxide promoters.

THE CATALYST SUPPORT

The catalytically active metals, the alkali, and the promoter metaloxide, if present, are distended on alumina. Although other supports maybe used, it has been found, for example, that silica, titania, chromia,magnesia, silica-alumina and zirconia produce catalysts with much loweractivities.

To be most effective when used as a support, alumina should becharacterized by low acidity, high surface area, and high purity. Theseproperties are necessary in order to enable the catalyst to have highactivity and a low deactivation rate, and to produce high molecularweight hydrocarbon products. The surface area of the alumina support isat least, and preferably greater than, about 100 m² /g; and morepreferably at least about or greater than 150 m² /g. The pore volume isat least, and preferably greater than, about than 0.3 cm³ /g. Thecatalyst support must be of high purity. That is, the content ofelements, e.g. sulfur and phosphorous, that have a deleterious effect oncatalytic activity must be kept low. The sulfur content of the catalystsupport should be kept below 100 ppm and preferably below 50 ppm.Although gamma alumina has generally been used and is preferred, anumber of alumina structures, if prepared properly, can meet theseconditions and are suitable supports. For example, etaalumina,xi-alumina, theta-alumina, delta-alumina, kappa-alumina, boehmite andpseudo-boehmite can all be used as supports.

CATALYST PREPARATION

The method of depositing the active metals, the alkali and the promoteroxide on the alumina support is not critical, and can be chosen fromvarious methods well known to those skilled in the art. One suitablemethod that has been employed is known as incipient wetnessimpregnation. In this method the metal salts are dissolved in an amountof a suitable solvent just sufficient to fill the pores of the catalyst.In another method, the metal oxides or hydroxides are coprecipitatedfrom an aqueous solution by adding a precipitating agent. In stillanother method, the metal salts are mixed with the wet support in asuitable blender to obtain a substantially homogeneous mixture. In thepresent invention, if incipient wetness impregnation is used, thecatalytically active metals and the alkali can be deposited on thesupport using an aqueous or an organic solution. Suitable organicsolvents include, for example, acetone, methanol, ethanol, dimethylformamide, diethyl ether, cyclohexane, xylene and tetrahydrofuran.Aqueous impregnation is preferred when Co(No₃)₂ is used as the salt,while an organic solvent is the preferred solvent when the catalyst isprepared from cobalt carbonyl.

Suitable cobalt compounds include, for example, cobalt nitrate, cobaltacetate, cobalt chloride and cobalt carbonyl, with the nitrate being themost preferable when impregnating from an aqueous solution. Suitablerhenium compounds include, for example, rhenium oxide, rhenium chlorideand perrhenic acid. Perrhenic acid is the preferred compound whenpreparing a catalyst using an aqueous solution. Suitable alkali saltsfor incorporating the alkali into the catalyst include, for example, thenitrates, chlorides, carbonates, and hydroxides. The metal oxidepromoter can suitably be incorporated into the catalyst in the form, forexample, of the nitrate or chloride.

After aqueous impregnation, the catalyst is dried at 110° to 120° C. for3 to 6 hours. When impregnating from organic solvents, the catalyst ispreferably first dried in a rotary evaporator apparatus at 50° to 60° C.under low pressure, then dried at 110° to 120° C. for several hourslonger.

The dried catalyst is calcined under flowing air by slowly increasingthe temperature to an upper limit of between 200° and 500° C.,preferably between 250° and 350° C. The rate of temperature increase ispreferably between 0.5° and 2° C. per minute, and the catalyst is heldat the highest temperature for a period of 2 to 5 hours. Theimpregnation procedure is repeated as many times as necessary to obtaina catalyst with the desired metals content. Cobalt, rhenium, alkali andthe metal oxide promoter, if present, can be impregnated together, or inseparate steps. If separate steps are used, the order of impregnatingthe active components can be varied.

Before use, the calcined catalyst is preferably reduced with hydrogen.This can suitably be done in flowing hydrogen at atmospheric pressure ata flow rate between 30 and 100 cm³ /min when reducing about 2 g ofcatalyst. The flow rate should suitably be increased for largerquantities of catalyst. The temperature is increased at a rate between0.5° and 2° C. per minute from ambient to a maximum level of 250° to450° C., preferably between 300° and 400° C., and maintained at themaximum temperature for about 6 to 24 hours, more preferably 10 to 24hours.

After the reduction step, the catalyst may be oxidized and rereducedbefore use. To carry out the oxidation step, the catalyst is treatedwith dilute oxygen (1-3% oxygen in nitrogen) at room temperature for aperiod of 1/2 to 2 hours before the temperature is increased at the samerate and to the same temperature as used during calcination. Afterholding the high temperature for 1 to 2 hours, air is slowly introduced,and the treatment is continued under air at the high temperature foranother 2 to 4 hours. The second reduction is carried out under the sameconditions as the first reduction.

HYDROCARBON SYNTHESIS

The reactor used for the synthesis of hydrocarbons from synthesis gascan be chosen from various types well known to those skilled in the art,for example, fixed bed, fluidized bed, ebullating bed or slurry. Thecatalyst particle size for the fixed or ebullating bed is preferablybetween 0.1 and 10 mm and more preferably between 0.5 and 5 mm. For theother types of operations a particle size between 0.01 and 0.2 mm ispreferred.

The synthesis gas is a mixture of carbon monoxide and hydrogen and canbe obtained from any source known to those skilled in the art, such as,for example, steam reforming of natural gas or partial oxidation ofcoal. The molar ratio of H_(2:) CO is preferably between 1:1 to 3:1; andmore preferably between 1.5:1 to 2.5:1. Carbon dioxide is not a desiredfeed component for use with the catalyst of this invention, but it doesnot adversely affect the activity of the catalyst. All sulfur compoundsmust, on the other hand, be held to very low levels in the feed,preferably below 1 ppm.

The reaction temperature is suitably between 150° and 300° C.; and morepreferably between 175° and 250° C. The total pressure can be fromatmospheric to around 100 atmospheres, preferably between 1 and 30atmospheres. The gaseous hourly space velocity, based on the totalamount of synthesis gas feed, is preferably between 100 and 20,000 cm³of gas per gram of catalyst per hour; and more preferably from 1000 to10,000 cm³ /g/h, where gaseous hourly space velocity is defined as thevolume of synthesis gas (measured at standard temperature and pressure)fed per unit weight of catalyst per hour.

The reaction products are a complicated mixture, but the main reactioncan be illustrated by the following equation:

    nCO+2nH.sub.2 →(--CH.sub.2 --).sub.n +nH.sub.2 O

where (--CH₂ --)_(n) represents a straightchain hydrocarbon of carbonnumber n. Carbon number refers to the number of carbon atoms making upthe main skeleton of the molecule. In F-T synthesis, the products aregenerally either paraffins, olefins, or alcohols. Products range incarbon number from one to 50 or higher.

In addition, with many catalysts, for example those based on iron, thewater gas shift reaction is a well known side reaction:

    CO+H.sub.2 O→H.sub.2 +CO.sub.2

With cobalt catalysts the rate of this last reaction is usually verylow. However, it is found that, even though rhenium catalysts exhibit arelatively high selectivity to carbon dioxide, the cobalt plus rheniumcatalyst of this invention surprisingly does not have a higherselectivity to carbon dioxide than the cobalt only catalyst.

The hydrocarbon products from Fischer-Tropsch synthesis are distributedfrom methane to high boiling compounds according to the so calledSchulz-Flory distribution, well known to those skilled in the art. TheSchulz-Flory distribution is expressed mathematically by theSchulz-Flory equation:

    W.sub.i =(1-α).sup.2 iα.sup.i-l

where i represents carbon number, α is the Schulz-Flory distributionfactor which represents the ratio of the rate of chain propagation tothe rate of chain propagation plus the rate of chain termination, andW_(i) represents the weight fraction of product of carbon number i. Thisequation shows that an increased α results in a higher average carbonnumber of the products. Higher α values are desirable when heavierproducts, such as diesel fuel, are relatively more valuable than lighterproducts, such as naphtha.

The products produced by the catalyst of this invention generally followthe Schulz-Flory distribution, except that the yield of methane isusually higher than expected from this distribution. This indicates thatmethane is apparently produced by an additional mechanism.

Catalysts promoted with alkali in the manner prescribed in thisinvention produce a product having a higher average carbon number thanthe products from non-alkali promoted catalysts. That is, theSchulz-Flory α for the product from an alkali promoted catalyst ishigher than for a non-alkali promoted catalyst.

It is well known, and also shown in one of the following examples, thatrhenium alone is a low activity catalyst for Fischer-Tropsch synthesis,producing a product which is predominantly methane, and that the alkalishave no activity for F-T synthesis. On the other hand, cobalt is a wellknown catalyst for producing higher carbon number hydrocarbons. In U.S.Pat. No. 4,568,663, it has been shown that adding small amounts ofrhenium to cobalt supported on titania improves the catalytic activity.In the present invention, it has been found that the hydrocarbon yieldobtained by adding rhenium is surprisingly much larger for an aluminasupported cobalt catalyst than that obtained from cobalt and rhenium onseveral other inorganic supports. The improved activity and increasedselectivity to heavier hydrocarbons is followed by no deleterious effecton the selectivity to methane.

The catalyst of this invention is further described in the followingexamples.

EXPERIMENTAL WORK

The following examples describe the preparation of various catalysts andthe results obtained from testing these catalysts for conversion ofsynthesis gas into hydrocarbons.

Before being tested, each catalyst was given a pretreatment consistingof reduction by passing hydrogen over the catalyst at a rate of 3000 cm³/g/h while heating the catalyst at a rate of 1° C./min to 350° C. andmaintaining this temperature for 10 hours. In the tests, synthesis gasconsisting of 33 vol % carbon monoxide and 67 vol % hydrogen was passedover 0.5 g of the catalyst at atmospheric pressure at temperatures of185°, 195° and 205° C. according to the following schedule:

9 hr. 50 min. at 195° C.

4 hr. 20 min. at 205° C.

4 hr. 30 min. at 185° C.

9 hr. 50 min. at 195° C.

The flow rate of synthesis gas was 1680 cm³ /g of catalyst/h. Productsfrom the reactor were sent to a gas chromatograph for analysis.Catalysts were compared based on the results over the period from 10 to30 hours on stream.

EXAMPLE 1 Catalyst Containing Cobalt But No Rhenium or Alkali

This example describes the preparation of a control cobalt catalystwhich was used for comparative purposes. This catalyst was prepared asfollows:

A solution was prepared by dissolving 17.03 g of cobalt nitrate,Co(NO₃)₂.6H₂ O, and 0.76 g of mixed rare earth nitrate, RE(NO₃)₃, whereRE stands for rare earth with a composition of 66% La₂ O₃, 24% Nd₂ O₃,8.2% Pr₆ O₁₁, 0.7% CeO₂, and 1.1% other oxides (Molycorp 5247), in 30 mlof distilled water. The total solution was added with stirring to 25 gof Ketjen CK300 gammaalumina which had been calcined 10 hours at 500° C.The prepared catalyst was then dried for 5 hours in an oven at atemperature of 115° C. The dried catalyst was then calcined in air byraising its temperature at a heating rate of 1° C./minute to 300° C. andholding at this temperature for 2 hours. The finished catalyst contained12 wt % cobalt and 1 wt % rare earth oxide with the remainder beingalumina. This catalyst is referred to as preparation "a" in Table I. Theabove procedure was repeated to produce preparation "b" catalyst inTable I.

The results of the tests with this catalyst are shown in Table I. Inthis and the following tables, selectivity is defined as the percent ofthe carbon monoxide converted that goes to the indicated product.

                  TABLE I                                                         ______________________________________                                              Pre-   CO        C.sub.2 +                                                                             CH.sub.4                                                                              CO.sub.2                               Temp. par-   Conversion                                                                              Selectivity                                                                           Selectivity                                                                           Selectivity                            °C.                                                                          ation  %         %       %       %                                      ______________________________________                                        185   a       7        91.1    7.2     1.7                                          b      11        91.8    7.1     1.1                                    195   a      12        90.0    8.9     1.1                                          b      18        90.2    9.0     0.8                                    205   a      21        87.7    11.3    1.0                                          b      29        86.7    12.4    0.9                                    ______________________________________                                    

This example shows that a cobalt catalyst exhibits good selectivity toethane and longer chain length hydrocarbons and low selectivity tomethane and carbon dioxide.

EXAMPLE 2 Catalyst Containing Rhenium But No Cobalt or Alkali

This example describes a rhenium catalyst prepared for comparativepurposes. The procedure employed was the same as for Example 1 exceptthat the solution contained 0.33 g of perrhenic acid, HReO₄ as 82.5%aqueous solution, and 0.54 g of rare earth nitrate to make 24 ml ofsolution which then was added to 20 g of calcined alumina. The finishedcatalyst contained 1 wt % rhenium and 1 wt % rare earth oxide with theremainder being alumina.

The results of the tests with the catalyst of Example 2 are shown inTable II.

                  TABLE II                                                        ______________________________________                                              CO         C.sub.2 +  CH.sub.4 CO.sub.2                                 Temp. conversion Selectivity                                                                              Selectivity                                                                            Selectivity                              °C.                                                                          %          %          %        %                                        ______________________________________                                        185   0.3        20         30       50                                       195   0.3        19         31       50                                       205   0.3        19         31       50                                       ______________________________________                                    

EXAMPLE 3 Catalyst Containing Rhenium But No Cobalt or Alkali

Repetition of the procedure from Example 2, except that 0.83 g ofperrhenic acid were used, gave a catalyst containing 4 wt % rhenium. Theresults of the tests with the catalyst of Example 3 are shown in TableIII.

                  TABLE III                                                       ______________________________________                                              CO         C.sub.2 +  CH.sub.4 CO.sub.2                                 Temp. conversion Selectivity                                                                              Selectivity                                                                            Selectivity                              °C.                                                                          %          %          %        %                                        ______________________________________                                        185   0.3        20         30       50                                       195   0.3        19         31       50                                       205   0.3        19         31       50                                       ______________________________________                                    

The results from Examples 2 and 3 show that catalysts containing rheniumbut no cobalt have very low activity for producing desirable liquidhydrocarbons from synthesis gas. Furthermore, about half the product iscarbon dioxide, and most of the hydrocarbon product is methane.

EXAMPLES 4 THROUGH 11 Catalysts Containing Both Cobalt And Rhenium ButNo Alkali

The preparation procedure of Example 1 was employed except that varyingamounts of perrhenic acid were added to the solution. This produced aseries of catalysts containing 12 wt % cobalt and 0.1, 0.2, 0.3, 0.5,1.0, 2.0, 4.0, and 8.0 wt % rhenium in addition to 1.0 wt % rare earthoxide.

The results of the tests with the catalysts of Examples 4 through 11 at195° C. are shown in Table IV and further illustrated in FIG. 1. FIG. 1shows the effect on carbon monoxide conversion of adding rhenium tocatalysts containing 12% cobalt.

                  TABLE IV                                                        ______________________________________                                        Exam- Co    Re    CO     C.sub.2 +                                                                             CH.sub.4                                                                              CO.sub.2                             ple   wt    wt    conver-                                                                              Selectivity                                                                           Selectivity                                                                           Selectivity                          No.   %     %     sion % %       %       %                                    ______________________________________                                        4     12    0.1   26     89.8     9.6    0.6                                  5     12    0.2   29     88.9    10.4    0.7                                  6     12    0.3   27     88.2    11.0    0.8                                  7     12    0.5   31     88.3    10.9    0.8                                  8     12    1.0   33     87.7    11.4    0.9                                  9     12    2.0   31     85.7    13.3    1.0                                  10    12    4.0   28     84.7    14.2    1.1                                  11    12    8.0   25     84.5    14.2    1.3                                  ______________________________________                                    

As can be seen from comparison of the results in Table I with Table IVand FIG. 1, the addition of small amounts of rhenium to a cobaltsupported on alumina catalyst significantly increases the conversion ofthe carbon monoxide in the feed. Levels of rhenium as low as 0.1 wt %result in approximately doubling the CO conversion. The exact level ofRe for optimum activity is very important, as the rate of carbonmonoxide conversion increases rapidly at low rhenium addition levels,reaches a maximum and then decreases gradually at levels greater than 1wt % rhenium. However, even at the highest rhenium level investigated(8%), a clear improvement in conversion is evident when compared to thecatalyst not containing rhenium.

It is important that the increase in activity occur without acorresponding increase in either the methane or the carbon dioxideselectivities. Table IV shows that the increase in carbon monoxideconversion is not accompanied by any substantial change in either theselectivities to methane or carbon dioxide. Thus, after rhenium additionthe principal reaction products are still desirable hydrocarbons.

EXAMPLES 12 THROUGH 25 Catalysts Containing Both Cobalt and Rhenium butNo Alkali

The preparation procedure of Example 1 was employed except that varyingamounts of cobalt nitrate and perrhenic acid were added to the solution.This produced a series of catalysts containing from 3.0 to 40 wt %cobalt and from 0 to 5.0 wt % rhenium in addition to 1.0 wt % rare earthoxide.

The results of the tests with the catalysts of Examples 12 through 25 at195° C. are shown in Table V.

                  TABLE V                                                         ______________________________________                                        Ex-                                                                           am-  Co    Re     CO     C.sub.2 +                                                                             CH.sub.4                                                                              CO.sub.2                             ple  wt    wt     Conver-                                                                              Selectivity                                                                           Selectivity                                                                           Selectivity                          No.  %     %      sion % %       %       %                                    ______________________________________                                        12    3    0.0    5      90.7    8.1     1.2                                  13    3     0.25  4      87.2    10.4    2.4                                  14    6    0.0    12     90.0    8.9     1.1                                  15    6    0.5    16     88.2    10.8    1.0                                  16    9    0.0    15     90.0    9.1     0.9                                  17    9     0.75  25     88.1    11.1    0.8                                  18   20    0.0    20     89.3    9.8     0.9                                  19   20    0.5    40     87.9    11.1    1.0                                  20   20    1.0    46     86.1    12.9    1.0                                  21   20    5.0    42     83.9    14.8    1.3                                  22   40    0.0    20     89.3    9.7     1.0                                  23   40    1.0    56     85.0    13.2    1.8                                  24   40    2.0    58     84.3    13.7    2.0                                  25   40    5.0    60     81.9    15.7    2.4                                  ______________________________________                                    

The results in Table V show that for cobalt catalysts without rhenium,there is a significant increase in activity in going from 3% cobalt to6% cobalt. However, only modest increases in activity occur from thispoint up to cobalt loadings of as high as 40%. At a cobalt loading of3%, the addition of rhenium does not improve the catalytic activity, butthe improvment upon rhenium addition is significant for higher cobaltloadings. In fact, the improvement in activity due to the addition ofrhenium increases as the cobalt content increases as shown in FIG. 2.

EXAMPLES 26 AND 27 Cobalt/Rhenium Catalysts with Promoters

To illustrate the use of promoters other than rare earth oxides, thefollowing catalysts were prepared. The preparation procedure used toprepare the catalyst of Example 8 was used except that zirconiumnitrate, Zr(NO₃)₄, or vanadyl oxalate, VO(C₂ O₄ H)₃, was substituted forthe rare earth nitrate. The results of tests at 195° C. with thecatalysts of Examples 26 and 27 are shown in Table VI. In addition tothe promoter, these catalysts contained 12% cobalt and 1% rhenium andwere supported on alumina.

                  TABLE VI                                                        ______________________________________                                                                     C.sub.2 +                                                                           CH.sub.4                                                                            CO.sub.2                                                 CO       Selec-                                                                              Selec-                                                                              Selec-                               Example             conver-  tivity                                                                              tivity                                                                              tivity                               No.    Promoter     sion %   %     %     %                                    ______________________________________                                        26     ZrO.sub.2 (0.75 wt %)                                                                      31       87.9  11.3  0.8                                  27     V.sub.2 O.sub.5 (0.56 wt %)                                                                26       89.4   9.8  0.8                                  ______________________________________                                    

EXAMPLES 28 THROUGH 41 Cobalt/Rhenium Catalysts On Other Supports

For comparison with alumina, several catalysts were prepared on othersupports. The preparation procedure used to prepare the catalyst ofExample 8 was repeated, but without the addition of rare earth oxide.The titanium-supported catalysts were prepared on titania calcined atboth 500° C. and 600° C. After calcination at 600° C., the titania ismainly in the crystalline rutile form; while after calcination at 500°C. the anatase:rutile ratio is about 1:1. The catalysts prepared on thetitania support calcined at these two temperatures showed exactly thesame catalytic activity.

The supports used were: Davison Grade 59 silica; Degussa P25 titania;Alpha Chemicals No. 88272 chromia; magnesia prepared by calciningFischer basic magnesium carbonate; American Cyanamid AAA Silica-Alumina;and Alpha Chemicals 11852 zirconia (containing 2% alumina). Informationon the composition of the catalysts prepared on the different supportsis given in Table VII.

                                      TABLE VII                                   __________________________________________________________________________                      Weight of Materials in                                                                    Composition of                                              Weight of                                                                           Impregnating                                                                              Finished                                        Example     Support                                                                             Solution, g Catalyst, wt %                                  No.   Support                                                                             g     Co(NO.sub.3).sub.2                                                                  HReO.sub.4 *                                                                        Co  Re                                          __________________________________________________________________________    28    Silica                                                                              20    13.47 --    12  --                                          29    Silica                                                                              20    13.62 0.38  12  1.0                                         30    Titania**                                                                           25    16.84 --    12  --                                          31    Titania**                                                                           24.64 16.78 0.46  12  1.0                                         32    Titania***                                                                          25    16.84 --    12  --                                          33    Titania***                                                                          24.64 16.78 0.46  12  1.0                                         34    Chromia                                                                             20    13.47 --    12  --                                          35    Chromia                                                                             21.3  14.51 0.40  12  1.0                                         36    Magnesia                                                                            21.59 14.54 --    12  --                                          37    Magnesia                                                                            14.54 10.67 0.29  12  1.0                                         38    Silica-                                                                       Alumina                                                                             20    13.47 --    12  --                                          39    Silica-                                                                       Alumina                                                                             20    13.62 0.38  12  1.0                                         40    Zirconia                                                                            20    13.47 --    12  --                                          41    Zirconia                                                                            20    13.62 0.38  12  1.0                                         __________________________________________________________________________     *Weight of 82.5% perrhenic acid solution.                                     **Calcined at 500° C.                                                  ***Calcined at 600° C.                                            

A series of tests was conducted to evaluate the activities of thecatalysts of the above examples in converting synthesis gas intohydrocarbons. The results of the tests with the catalysts of Examples 28through 41 at 195° C. are shown in Table VIII. The results fromcatalysts prepared on alumina are included for comparison.

                                      TABLE VIII                                  __________________________________________________________________________                     CO    C.sub.2 +                                                                          CH.sub.4                                                                            CO.sub.2                                    Example                                                                            Co Re       Conversion                                                                          Selec-                                                                             Selec-                                                                              Selec-                                      No.  %  %  Support                                                                             %     tivity %                                                                           tivity %                                                                            tivity %                                    __________________________________________________________________________    1    12 -- Al.sub.2 O.sub.3                                                                    12    90.0 8.9   1.1                                         8    12 1  Al.sub.2 O.sub.3                                                                    33    87.7 11.4  0.9                                         28   12 -- SiO.sub.2                                                                           11    90.1 8.7   1.2                                         29   12 1  SiO.sub.2                                                                           12    88.1 10.7  1.2                                         30   12 -- TiO.sub.2 *                                                                         11    87.6 11.8  0.6                                         31   12 1  TiO.sub.2 *                                                                         17    86.5 12.8  0.7                                         32   12 -- TiO.sub.2 **                                                                        11    87.6 11.7  0.7                                         33   12 1  TiO.sub.2 **                                                                        17    85.8 13.5  0.7                                         34   12 -- Cr.sub.2 O.sub.3                                                                    1     83.5 15.5  1.0                                         35   12 1  Cr.sub.2 O.sub.3                                                                    2     80.8 12.3  6.9                                         36   12 -- MgO   0.3   20.0 30.0  50.0                                        37   12 1  MgO   0.3   19.1 30.9  50.0                                        38   12 -- SiO.sub.2 /Al.sub.2 O.sub.3                                                         5     76.3 22.2  1.5                                         39   12 1  SiO.sub.2 /Al.sub.2 O.sub.3                                                         6     78.6 19.8  1.6                                         40   12 -- ZrO.sub.2                                                                           4     80.9 16.3  2.8                                         41   12 1  ZrO.sub.2                                                                           7     78.8 18.7  2.5                                         __________________________________________________________________________     *Support calcined at 500° C.                                           **Support calcined at 600° C.                                     

The catalysts in Table VII were prepared to test the teaching thatvarious inorganic supports are acceptable for preparing cobalt plusrhenium F-T catalysts. An examination of the data in Table VIII leads tothe surprising conclusion that the type of support is extremelyimportant and that vast differences in activity exist between catalystsprepared on one support and catalysts of the same catalytic metalscontent on another support. More surprisingly, only cobalt plus rheniumon alumina showed a commercially attractive activity level andselectivity.

Catalysts on magnesia and chromia exhibited extremely low activities,both with and without rhenium. Catalysts on zirconia and silica-aluminashowed somewhat higher activities, but selectivity to C₂ + hydrocarbonswas poor. These catalysts showed only modest improvements in activityupon the addition of rhenium.

Catalysts without rhenium supported on silica and titania showedactivity levels close to comparable cobalt on alumina catalyst. However,upon addition of rhenium, the alumina catalyst showed a surprisingincrease in activity from about 15% carbon monoxide conversion to 33%carbon monoxide conversion; whereas, the silica supported catalystshowed only a very small increase in activity from 11% carbon monoxideconversion to 12% carbon monoxide conversion, while the titaniasupported catalyst showed a larger, but still modest, gain in activityfrom 11% carbon monoxide conversion to 17% carbon monoxide conversion.

From these examples, plus those presented previously, it can beconcluded that the catalytic activity of a cobalt catalyst supported onalumina is greatly improved by adding minor amounts of rhenium, as longas the cobalt level is greater than about 5 wt %. Although improvedactivity from rhenium addition is also observed for some other supports,the activity level achieved by adding rhenium to a catalyst supported onalumina is much higher than for other supports. This result issurprising and would not have been predicted based on teachings in theprior art.

EXAMPLES 42 THROUGH 56 Catalyst Containing Cobalt, Rhenium, and Alkali

To demonstrate the advantages of including an alkali in the catalysts ofthis invention, the catalysts of Examples 42 through 55 were preparedand tested.

The catalysts of Examples 42 to 50 were prepared on Harshaw Al 4100Palumina which had been screened to 100-270 mesh. The catalysts ofExamples 51 to 56 were prepared on Ketjen CK300 gamma alumina, screenedto 20-40 mesh. Both supports were calcined overnight at 500° C. beforeuse. The technique used for preparing the catalysts was incipientwetness, as described in Example 1. The amounts of materials used in thepreparation of each catalyst are shown in Table IX. The preparedcatalysts were then air dried for 5 to 24 hours in an oven at atemperature of 120° C. The dried catalysts were then calcined in air byraising the temperature at a heating rate of 1° C./minute to 300° C. andholding at this temperature for 2 to 16 hours. The compositions of thefinished catalysts are shown in Table IX.

The results from testing these catalysts are shown in Table X. As isclear from these results, the addition of an alkali to the catalystserves to increase the average molecular weight of the product, asevidenced by an increase in the Schulz-Flory α. Higher levels of thealkali result in higher α's as illustrated in FIG. 3. However, activitydecreases as the alkali content increases, as illustrated in FIG. 4.Therefore, for any particular situation there is an optimum alkali levelthat balances desired average product moleular weight and catalystacitvity. Also, the effectiveness of the alkali varies from one alkalito another with, for example, potassium being more effective thanlithium.

                                      TABLE IX                                    __________________________________________________________________________                               Composition of                                          Wt of                                                                             Type                                                                              Weight of Material in                                                                       Finished Catalyst                                  Example                                                                            Al.sub.2 O.sub.3                                                                  of  Impregnation solution, g                                                                    wt %                                               No.  g   Alkali                                                                            (a)  (b)                                                                              (c)                                                                              (d)                                                                              Co                                                                              Re REO                                                                              Alkali                                     __________________________________________________________________________    42   300.0                                                                             --  1039.65                                                                            17.21                                                                            13.99                                                                            -- 40                                                                              2.0                                                                              1.0                                                                              0.0                                        43   75.0                                                                              K   260.46                                                                             4.31                                                                             3.50                                                                             0.34                                                                             40                                                                              2.0                                                                              1.0                                                                              0.1                                        44   75.0                                                                              K   256.36                                                                             4.25                                                                             -- 0.67                                                                             40                                                                              2.0                                                                              -- 0.2                                        45   175.0                                                                             K   609.03                                                                             10.08                                                                            8.20                                                                             1.59                                                                             40                                                                              2.0                                                                              1.0                                                                              0.2                                        46   100.0                                                                             K   349.50                                                                             5.79                                                                             4.70                                                                             1.83                                                                             40                                                                              2.0                                                                              1.0                                                                              0.4                                        47   100.0                                                                             K   354.03                                                                             5.86                                                                             4.76                                                                             4.64                                                                             40                                                                              2.0                                                                              1.0                                                                              1.0                                        48   60.0                                                                              Na  205.49                                                                             3.40                                                                             -- 0.93                                                                             40                                                                              2.0                                                                              -- 0.24                                       49   60.0                                                                              Cs  209.55                                                                             3.47                                                                             -- 2.12                                                                             40                                                                              2.0                                                                              -- 1.36                                       50   65.0                                                                              Rb  225.38                                                                             4.23                                                                             -- 1.71                                                                             40                                                                              2.0                                                                              -- 0.87                                       51   20.0                                                                              --  13.78                                                                              0.38                                                                             0.62                                                                             -- 12                                                                              1.0                                                                              1.0                                                                              --                                         52   20.0                                                                              K   13.80                                                                              0.38                                                                             0.62                                                                             0.06                                                                             12                                                                              1.0                                                                              1.0                                                                              0.1                                        53   20.0                                                                              Li  13.64                                                                              0.38                                                                             -- 0.11                                                                             12                                                                              1.0                                                                              -- 0.05                                       54   20.0                                                                              Li  13.80                                                                              0.38                                                                             0.62                                                                             0.12                                                                             12                                                                              1.0                                                                              1.0                                                                              0.05                                       55   20.0                                                                              Cs  13.67                                                                              0.38                                                                             -- 0.10                                                                             12                                                                              1.0                                                                              -- 0.3                                        56   20.0                                                                              Cs  13.83                                                                              0.38                                                                             0.62                                                                             0.10                                                                             12                                                                              1.0                                                                              1.0                                                                              0.3                                        __________________________________________________________________________     (a) Co(NO.sub.3).sub.2.6H.sub.2 O                                             (b) 82.5% HReO.sub.4 solution, except for Example 50 which was 72.9%          HReO.sub.4?                                                                   (c) Rare earth nitrates (see Example 1)                                       (d) LiNO.sub.3, NaNO.sub.3, KNO.sub.3, RbNO.sub.3 or CsNO.sub.3          

                  TABLE X                                                         ______________________________________                                        Ex-   Alkali                  CH.sub.4                                        ample        Content, CO conversion                                                                           Selectivity                                                                           Product                               No.   Type   wt %     %         %       alpha*                                ______________________________________                                        42    --     --       52.5**    14.2**  0.75**                                43    K      0.1      52        8.6     0.86                                                        50        11.1    0.79                                  44    K      0.2      43        9.3     0.83                                  45    K      0.2      51        9.5     0.84                                                        44        11.6    0.84                                                        41        9.4     0.81                                  46    K      0.4      30        7.3     0.87                                                        35        7.8     0.85                                  47    K      1.0      12        7.0     0.88                                                        9         7.9     0.85                                                        6         6.2     0.87                                  48    Na     0.24     21        8.3     0.82                                  49    Cs     1.36     14        8.5     0.86                                  50    Rb     0.87     11        7.3     0.86                                  51    --     --       31.7***   10.9*** 0.77***                               52    K      0.1      19        7.6     0.78                                                        22        9.8     0.84                                  53    Li     0.05     31        11.7    0.78                                  54    Li     0.05     27        12.0    0.78                                  55    Cs     0.3      21        11.1    0.83                                  56    Cs     0.3      21        10.3    0.84                                  ______________________________________                                         *Calculated by plotting ln(W.sub.n /n) vs. n, where n is carbon number an     W.sub.n  is the weight fraction of the product having a carbon number n,      and determining the slope of the line.                                        **Average of 21 tests.                                                        ***Average of 6 tests.                                                   

What is claimed is:
 1. A catalyst for converting synthesis gas tohydrocarbons comprising amounts of cobalt catalytically active in aFischer-Tropsch synthesis, relatively lesser amounts of rhenium than thecobalt content and relatively lesser amounts of an alkali metal than thecobalt content of the catalyst, composited on an alumina support,wherein the alkali metal is selected from the group consisting ofelements from Group IA of the periodic table.
 2. The catalyst recited inclaim 1 wherein the alkali metal is present in amounts ranging fromabout 0.5 to 5 atom percent of the cobalt content of the catalyst. 3.The catalyst recited in claim 1 wherein the alkali metal is incorporatedinto the catalyst as a salt selected from the group consisting ofnitrates, chlorides, carbonates and hydroxides.
 4. The catalyst recitedin claim 1 wherein cobalt is present in amounts ranging from 5 to about60 wt % of the catalyst.
 5. The catalyst recited in claim 1 whereincobalt is present in amounts ranging from about 10 to about 40 wt % ofthe catalyst.
 6. The catalyst recited in claim 1 wherein rhenium ispresent in amounts ranging from about 0.5 to 50 wt % of the cobaltcontent of the catalyst.
 7. The catalyst recited in claim 1 whereinrhenium is present in amounts ranging from about 1 to 30 wt % of thecobalt content of the catalyst.
 8. The catalyst recited in claim 1wherein said support is gamma alumina.
 9. The catalyst recited in claim1 wherein said support has a surface area of at least about 100 m² /gand a pore volume of at least about 0.3 cm³ /g.
 10. The catalyst recitedin claim 1 further comprising an effective amount of a promoter selectedfrom the group consisting of oxides of the elements chosen from groupsIII B, IV B and V B of the periodic table, the lanthanides and theactinides, MgO and MnO, and mixtures thereof.
 11. The catalyst recitedin claim , 10 wherein said promoter is present in an amount ranging fromabout 0.1 to 5 wt % of the catalyst.
 12. A catalyst for convertingsynthesis gas to hydrocarbons comprising cobalt, rhenium and an alkalimetal composited on an alumina support wherein cobalt is present incatalytically active amounts up to about 60 wt % of the catalyst,rhenium is present in amounts from about 0.5 to about 50 wt % of thecobalt content of the catalyst and the alkali metal is selected from thegroup consisting of elements from Group IA of the Periodic Table and ispresent in amounts from about 0.5 to 5 atom percent of the cobaltcontent of the catalyst.
 13. The catalyst recited in claim 13 furthercomprising an effective amount of a promoter selected from the groupconsisting of oxides of the elements chosen from Groups III B, IV B andV B of the periodic table, the lanthanides and the actinides, MgO andMnO, and mixtures thereof.
 14. The catalyst recited in claim 12 whereinsaid support has a surface area of at least about 150 m² /g and a porevolume of at least about 0.3 cm³ /g.
 15. The catalyst recited in claim12 wherein the sulfur content of the alumina support is kept to levelsbelow about one hundred parts per million.